It is often desirable to measure the average power of a particular signal. In the past, average signal power has been measured with diode detector circuits, particularly when the signal of interest is an a.c. signal in the radio-frequency range. In a diode detector circuit, the signal to be measured is placed across a diode while the diode is biased to be within the "square-law" region. In the square-law region, the voltage across the diode is proportional to the input signal power. As long as the input signal does not drive the diode out of the square law region, the signal power will be accurately measured. However, when the signal to be measured is complex, e.g., when the signal has multiple sinusoidal components, the respective sinusoidal components can constructively interfere, driving the diode out of the square-law region. This results in an inaccurate power measurement by the detector circuit. Other detector circuit inaccuracies occur with changes in ambient temperature, deleteriously affecting the power-measurement ability of the circuit.
In U.S. Pat. No. 5,376,880, the disclosure of which is incorporated herein by reference, a novel power detection circuit is disclosed which overcomes the disadvantages of the above diode power detector circuits. In the '880 patent, average power is measured, in an exemplary embodiment, through a circuit that employs first and second thermal detectors, an error amplifier, and a summer. The first thermal detector provides a reference signal to the error amplifier. The summer receives an input signal to be measured and the signal output from the error amplifier. The summer feeds a composite signal, that is, a signal indicative of the sum of the power of the input signal and the output signal from the error amplifier, to a second thermal detector. The second thermal detector receives the composite signal and outputs a feedback signal, indicative of the power of the composite signal, to the error amplifier. The error amplifier subtracts the feedback signal from the reference signal to produce an amplifier output signal indicative of the power of the input signal. This signal is amplified and output to the summer.
In an exemplary embodiment, the thermal detector comprises a combination of a thermal radiating element and a thermal sensor. A resistor is advantageously employed as the thermal radiator while a thermistor is advantageously employed as the thermal sensor. Because the thermal detector's output signal is based on the interaction of the thermal radiator and the thermal sensor, it is important to ensure a precise and reproducible relationship between these components. The present invention relates to thermal detectors and low-cost methods for their construction that create such a precise and reproducible relationship between thermal radiator and thermal sensor components.